Glass-metal feedthrough

ABSTRACT

A glass-metal feedthrough consists of an external conductor, a glass material and an internal conductor. The internal conductor has a coefficient of expansion αinternal, the glass material has a coefficient of expansion αglass, and the external conductor has a coefficient of expansion αexternal. The coefficient of expansion of the internal conductor αinternal is greater than the coefficient of expansion of the glass material αglass and the coefficient of expansion of the external conductor αexternal is at least 2 ppm/K, such as at least 4 ppm/K, greater than the coefficient of expansion of the glass material αglass in the temperature range of 20° C. to the glass transformation temperature.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a glass-metal feedthrough, consisting of an external conductor, a glass material and an internal conductor and to the use thereof in a medical device, in particular in an implantable medical device and relates also to equipment, and devices having such glass-metal feedthroughs.

2. Description of the Related Art

Glass-metal feedthroughs are used in diverse areas of application in electrical engineering. They are used, for example, for sealing of electric conductors into housings for components in electronics and sensor technology. In this context, such components feature fusion seals of glasses with various metal materials. By fusing an internal conductor consisting of metal into a glass or glass ceramic material which is surrounded by an external conductor consisting of metal, a hermetically sealed feedthrough of a conductor in a housing can be provided. Particularly special glass-metal feedthroughs are such feedthroughs wherein the feedthrough itself or parts of the feedthrough come into contact with the human body. With such feedthroughs it is important that high corrosion resistance and long-term stability of all utilized components can be ensured. In particular, such feedthroughs should only emit limited amounts of Ni in order to prevent formation of allergic reactions. This is defined in the standards for the permissible limits of so-called nickel release. Reference is hereby made to DIN EN1811 and DIN EN12472.

In glass-metal feedthroughs, Fe—Ni, Fe—Ni—Co, Fe—Ni—Cr alloys are predominantly used for the feedthrough conductor. The advantage of these materials is their excellent adaptability of the thermal expansion to the sealing glasses. However, all of these materials have significant amounts of Ni in the base material. They are furthermore nickel coated to protect the materials against corrosion, which in turn causes release of undesirable amounts of nickel.

In order to prevent Ni release, it was provided in the current state of the art to coat the feedthrough conductor or, respectively, the internal conductor with a sufficiently thick gold layer to reduce nickel permeation. This however, had the disadvantage that prevention of nickel leakage was insufficient and in order to achieve satisfactory prevention, very thick Au-layers with a thickness of more than 2.5 μm were necessary.

As an alternative to a solution featuring coated conductors, DE 198 42 943 A1 suggests as the closest prior art, to use a tantalum internal conductor or alternatively a nickel-free, non-corrosive and ferritic high grade stainless steel, for example the steel according to US-Standard AISI 446 which is a non-corrosive heat-resistant ferritic chrome steel with aluminum addition. The coefficient of expansion of the ferritic high grade stainless steel AISI 446 is only slightly higher than that of common glasses. Therefore, there is a danger that, during cooling the internal conductor contracts more than the glass and the interface with the glass ruptures. An additional disadvantage of DE 198 42 943 A1 is, that the selection of Ni-free materials was limited, since a sufficiently tight seal of the feedthrough can only be provided if the coefficient of expansion of the internal conductor αinternal was less than the coefficient of expansion of the glass αglass.

A glass composition has become known from U.S. Pat. No. 3,770,568 A which can be used for hermetically sealed tantalum electrolytic capacitors and which comprises an effective chrome content in the glass composition.

What is needed in the art is a glass-metal feedthrough which permits a wider selection of materials for the internal conductor and an effectively sealed feedthrough.

In particular, internal conductors should be possible which, on the one hand, do not release nickel when in contact with the human body and, on the other hand, offer sufficient sealability.

SUMMARY OF THE INVENTION

An exemplary embodiment of a glass-metal feedthrough provided in accordance with the present invention is characterized in that the coefficient of expansion of the internal conductor α_(internal) is greater than the coefficient of expansion of the glass α_(glass) and in that the difference between the coefficient of expansion of the external conductor α_(external) and the coefficient of expansion of the glass α_(glass) is at least 2 ppm/K, such as at least 4 ppm/K in the temperature range of 20° C. to the glass transformation temperature, and in that the coefficient of expansion of the external conductor α_(external) is greater than the coefficient of expansion of the glass al.

Compressive pre-stressing of the external conductor upon the glass achieves that the glass also exerts positive pressure (joint pressure) upon the glass to the internal conductor interface. This results in the advantage that that pressure reliably maintains a tightly sealed connection. Moreover, a low height of the glass-metal feedthrough is achieved which can be used for diverse applications such as devices for monitoring of patients or arm bands in the leisure sector.

The restriction to a greater expansion of the pin as internal conductor relative to glass, and an expansion of the external conductor relative to glass of at least greater than 2 ppm/K is particularly effective on highly expandable materials for the pin, with an expansion of greater than 10 ppm/K. Hardly any good sealing glasses are available in this range.

It has been surprisingly found that, for example, use of ferritic high grade stainless steel with high chrome content of between 10.5 and 20%, such as a high grade stainless steel with a chrome content of 15 to and 17%, for example ferritic high grade stainless steel AISI 430 is possible. As an alternative to the ferritic high grade stainless steels other materials such as molybdenum, tungsten or platinum can also be used. These additional materials also have a low allergy potential. The inventive condition α_(internal)>α_(glass) for the coefficients of expansion must also apply to these materials, wherein α_(internal) is the coefficient of expansion of the internal conductor and α_(glass) is the coefficient of expansion of the glass.

A problem in the use of these steels is however, that their thermal coefficient of expansion α_(internal) at 11.6 to 11.5 10⁻⁶/K is higher than the coefficient of expansion of the glass α_(glass) which is in the range of 10.6 to 6.1 10⁻⁶/K. It has been surprisingly found that in spite of a greater α_(internal) of the conductor which is above that of α_(glass)—in contrast to the current state of the art which dictates that with glass-metal feedthroughs the thermal expansion of the internal conductor must not be greater than that of the used glass, in order to provide a sufficiently tight seal—a tight glazed seal is also provided in the event that α_(internal) is greater than α_(glass) if a positively high joint pressure that is applied by the external conductor upon the glass is provided. As was demonstrated on further examination, it is not sufficient for a tight seal if the joint pressure is merely positive; in fact a joint pressure greater than 30 MPa, such as greater than 50 MPa or greater than 100 MPa must be attained to achieve a reliable glazed seal.

Such a high joint pressure is provided in the case of α_(internal) being less than α_(glass), if sufficient compressive pre-stressing is applied upon the glass by the external conductor. The thus resulting joint pressure between glass and internal conductor then occurs during cooling of the feedthrough after sealing. If this joint pressure is clearly positive, that is greater than 30 MPa, greater than 50 MPa, or greater than 100 MPa, the transition between glass and metal—that is, the transition from glass to the internal conductor—remains closed and therefore tightly sealed, even though α_(internal) is less than α_(glass). The joint pressure is directly dependent on the expansion differential. Moreover, dependencies on the inventive geometry are also conceivable. The joint pressure is a surface pressure. The joint pressure expresses the force per unit surface area with which a first body presses upon a second body.

In order to generate the necessary joint pressure, the difference between the coefficient of expansion of the external conductor α_(external) and the coefficient of expansion of the glass is at least 2 ppm/K, such as at least 4 ppm/K, wherein the coefficient of expansion is greater than the coefficient of expansion of the glass α_(glass). In some embodiments, the glass α_(glass) of the internal conductor is selected in such a manner that the coefficient of expansion of the internal conductor α_(internal) is 1.1 times greater than the coefficient of expansion α_(glass) of the glass. In some embodiments, α_(internal) is in the range of 1.1·α_(glass) to 2·α_(glass). In order to apply the necessary pressure of the external conductor onto the glass material and to ensure a tight seal, it is provided that the external conductor consists of a nickel-free, non-corrosive, chemically stable steel (high grade stainless steel). The coefficient of expansion of the external conductor is greater than that of the glass in order to provide the necessary joint pressure.

In some embodiments, the external conductor is made of austenitic steel, such as high grade stainless steel 316L which is characterized by good sealability and a high coefficient of expansion.

In addition to the nickel-free ferritic high grade stainless steel, in particular the non-hardenable ferritic high grade steel with a high chrome content whose coefficient of expansion α_(internal) is greater than that of the glass material the use of molybdenum, tungsten or platinum for the internal conductor is also conceivable.

The coefficients of expansion may be selected so that a joint pressure at the internal conductor of at least 30 MPa, such as at least 50 MPa or at least than 100 MPa is provided.

In addition to the feedthrough, exemplary embodiments of the invention also provide for the use of the glass-metal feedthrough in implantable medical devices or equipment, as well as providing an element that can be inserted into, or attached to the human or animal body or biological living cells containing cell cultures, said element having a glass-metal feedthrough, wherein the external conductor and the internal conductor—at least in the regions which come into contact with the human or animal body—consist of metals having a reduced allergy potential. In some embodiments, the external conductor as well as the internal conductor consist of these metals.

The metal of the external and the internal conductor can come into contact with the human or animal body or the cell cultures and are characterized in that they do not release any nickel and/or chrome.

The metal of the external and internal conductor—at least in the surface areas which during operation come into contact with the human or animal body, or with biological cells of the cell culture—may consist of nickel-free and/or chrome-free steels.

Especially suitable as metals for the internal conductor—at least in the surface areas which during operation come into contact with the human or animal body, or with biological cells of the cell culture—are metals selected from the group of the ferritic high grade stainless steels, platinum, platinum/iridium, niobium, titanium, molybdenum, tungsten as well as combinations thereof. The ferritic high grade stainless steels comprise AISI 4xx-groups, for example AISI 430.

The metals for the internal conductor—at least in the surface areas which during operation come into contact with the human or animal body, or with biological cells of the cell culture—may be selected from the group:

AISI 316 L

AISI 430

AISI 630

as well as combinations thereof. Additional materials for the external conductors can be selected from the group of austenitic high grade stainless steels AISI 3xx or ferritic high grade stainless steels AISI 4xx.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of an exemplary embodiment provided in accordance with the present invention;

FIG. 2A is a flow chart of joint pressure of conductor and glass; and

FIG. 2B is a flow chart of joint pressure of conductor and glass.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic depiction of a cross section through an exemplary embodiment provided in accordance with the invention. In the glass-metal feedthrough through a housing 1 illustrated in FIG. 1 which may be made of aluminum, a feedthrough 3 is inserted into an opening 5 in housing 1. Feedthrough 3 includes a feedthrough conductor or respectively an internal conductor 7 which is inserted into an external conductor 9. External conductor 9 may also be referred to as base body and may consist of an austenitic high grade stainless steel with a high thermal coefficient of expansion. The external conductor may also be referred to as base body and is joined with the housing, by, for example, welding. The internal conductor is glazed centrally into the external conductor or feedthrough conductor 7. This is achieved in that internal conductor 7 in an insulating glass body 11, completely fills the space inside external conductor 9. The glass into which the internal conductor 7 is fused may be a bio-compatible glass.

Also illustrated in FIG. 1 is diameter d of feedthrough conductor 7, as well as the so-called hole diameter D of opening 5.

According to the invention it is provided that the glass has a coefficient of expansion α_(glass), that the internal conductor has coefficient of expansion α_(internal) and that the external conductor has a coefficient of expansion α_(external). The materials are selected in such a manner, that the coefficient of expansion of the internal conductor α_(internal) is greater than that of the glass α_(glass). The difference between the coefficient of expansion of the external conductor and the coefficient of expansion of the glass is at least 2 ppm/K, such as at least 4 ppm/K. The coefficient of expansion of the external conductor α_(external) in the temperature range of 20° C. to the transformation temperature is greater than the coefficient of expansion α_(glass). Thus, a joint pressure is provided at the internal conductor of at least 30 MPa, such as of at least 50 MPa or of at least 100 MPa.

If, as demanded by the invention, the coefficient of expansion of the internal conductor α_(internal) is greater than that of the glass α_(glass), it will become problematic to provide a sufficiently tightly sealed feedthrough. In such a case, a sufficiently tight seal is ensured only if the external conductor can generate a sufficiently high joint pressure upon the glass. The joint pressure is relevant to d/D according to the state of the art is illustrated in FIG. 2A. Kovar which, with an a of 7.3 to 6.6 10⁻⁶1/K is slightly above the value of the coefficient of expansion α_(glass) and which is not covered by the invention was used as the material for the feedthrough conductor. d indicates the diameter of the conductor and D the diameter of opening 5. d/D specifies the relationship between the diameter of conductor d and hole diameter D. Generally, small d/D values characterize the large gap between conductor and opening and large d/D values characterize a small gap between conductor and opening.

The feedthroughs according to FIG. 2A are hermetically sealed. They however have the disadvantage that Kovar alloys have a high nickel content, so that nickel could be released from the feedthrough conductors.

Thus, it is provided according to the present invention to replace the Kovar feedthrough conductor with a material which does not release nickel. Surprisingly it was found that a material suitable for this purpose is a ferritic, Ni-free high grade stainless steel, in particular AISI 430. The disadvantage with a Ni-free high grade stainless steel, for example AISI 430 is however, that α_(internal) is at 11.5·10⁻⁶K⁻¹ and is thus clearly above the coefficient of expansion α_(glass) of the glass material. The coefficient of expansion of the used glasses α_(glass) is namely in a range of 6.1·10⁻⁶ K⁻¹ to 10.6·10⁻⁶K⁻¹.

In order to achieve a pressure tight glazed seal with such a constellation, a sufficiently high joint pressure which is applied by the external conductor must be generated.

The joint pressure for feedthroughs of this type is illustrated in FIG. 2B and is relevant to d/D.

As can be seen from FIG. 2B, a sufficiently high joint pressure for the use of AISI 430 as material for the feedthrough component results, especially if the external conductor consists of an austenitic high grade stainless steel with a α_(external) of 18.3 10⁻⁶/K. Such composition provides a sufficiently high joint pressure to ensure a tight seal. The joint pressure of such a material combination is identified with reference numbers 100 and 200. The joint pressure increases with a larger d/D and thus small gap, to values of above 150 MPa.

Curves 300 and 400 describe the joint pressure for an AISI 430 feedthrough conductor, wherein the external conductor is AISI 430 or AISI 630 and wherein the glass has a coefficient of expansion of 10.6·10⁻⁶ and is thus within the range of the coefficient of expansion of the external conductor as well as that of the feedthrough conductor. Because of this, the necessary joint pressure cannot be generated. Such material combinations generally demonstrate a low joint pressure. Curves 300 and 400 progress flat with little influence of the diameter due to the diameter ratio.

The materials of the different curves in the diagram according to FIG. 2B “joint pressure over d/D” are specified in the table below:

Ex- Glass Con- ternal Tg Glass ductor External CTE in CTE Conductor CTE Curve material to TG ° C. to Tg Material to TG Symbol 200 AISI316L 18.3 525 10.6 AISI430 11.5 ● 100 AISI316L 18.4 565 6.1 AISI430 11.6 ▴ 600 AISI430 11.5 525 10.6 AISI430 11.5 ▪ 400 AISI430 11.6 565 6.1 AISI430 11.6

300 AISI630 11.4 525 10.6 AISI430 11.5 ♦ 500 AISI630 11.4 565 6.1 AISI430 11.6

The material components of curves 100, 300 show an especially high joint pressure, so that a hermetic seal of the feedthrough is provided.

The specified glass-metal feedthroughs can be used in implantable medical devices or equipment. They can be produced cost effectively and are characterized by very low Ni release. Because of the high joint pressure they moreover are hermetically sealed—in other words feature a helium leakage of less than 1·10⁻⁸ mbar/sec.

While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims. 

What is claimed is:
 1. A glass-metal feedthrough, consisting of: a glass material having a coefficient of expansion α_(glass); an external conductor having a coefficient of expansion α_(external) that is greater than the coefficient of expansion of the glass material α_(glass), a difference between the coefficient of expansion of the external conductor α_(external) and the coefficient of expansion of the glass material α_(glass) is at least 2 ppm/K in a temperature range of 20° C. to a glass transformation temperature of the glass material, the external conductor consisting of a nickel-free, non-corrosive, chemically stable austenitic steel; and an internal conductor having a coefficient of expansion α_(internal) that is greater than the coefficient of expansion of the glass material α_(glass), the internal conductor directly contacting the glass material and consisting of one of the following materials: steel; stainless steel; nickel-free ferritic stainless steel; molybdenum; tungsten; platinum; niobium; titanium; or platinum and iridium; wherein the coefficient of expansion of the internal conductor α_(internal) and the coefficient of expansion of the external conductor αexternal are such that a joint pressure on the internal conductor of at least 30 MPa is generated in the temperature range of 20° C. to the glass transformation temperature of the glass material.
 2. The glass-metal feedthrough of claim 1, wherein the coefficient of expansion of the internal conductor αinternal is 1.1 times to 4 times greater than the coefficient of expansion of the glass material α_(glass).
 3. An element for insertion into or attachment to a human or animal body or biological living cells containing cell cultures, said element comprising: a glass-metal feedthrough comprising: a glass material having a coefficient of expansion α_(glass); an external conductor having a coefficient of expansion α_(external) that is greater than the coefficient of expansion of the glass material α_(glass), a difference between the coefficient of expansion of the external conductor α_(external) and the coefficient of expansion of the glass material α_(glass) is at least 2 ppm/K in a temperature range of 20° C. to a glass transformation temperature of the glass material, the external conductor consisting of an austenitic steel; and an internal conductor having a coefficient of expansion α_(internal) that is greater than the coefficient of expansion of the glass material α_(glass), wherein the external conductor and the internal conductor consist of metals which are configured to come into contact with the human or animal body or the cell cultures and do not release at least one of any nickel or any chrome, the internal conductor directly contacting the glass material and consisting of one of the following materials: steel; stainless steel; ferritic stainless steels; platinum; platinum and iridium; niobium; titanium; molybdenum; tungsten; and combinations thereof; wherein the coefficient of expansion of the internal conductor α_(internal) and the coefficient of expansion of the external conductor α_(external) are such that a joint pressure on the internal conductor of at least 30 MPa is generated in the temperature range of 20° C. to the glass transformation temperature of the glass material.
 4. The element of claim 3, wherein the metal of the internal conductor consist of at least one of nickel-free steel or chrome-free steel at least in surface areas which are configured to come into contact with the human or animal body or with biological cells of the cell culture in their operational use. 